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GG8.06 - The Unified Contact Angle Model (UCAM) for Fabricating Designer Surfaces for Quantitative Control of Contact Angles and Wetting Behavior 
April 10, 2015   10:30am - 11:00am

Reentrant features on a surface are key to render it omniphobic, i.e., the apparent contact angle, ?r > 90�, especially when the intrinsic contact angle, ?o < 90�. However, a complete theoretical understanding of the wetting behavior of surfaces with reentrant features has remained unclear. Here, we present a unified and general model that can predict apparent contact angles for surfaces with reentrant features also. Unlike previous models, this model does not include the �roughness� of surfaces, but rather the specific geometry of the surface features (protrusions, cavities, etc.) to determine the apparent contact angle in terms of the �intrinsic� or �Young� contact angle (on a smooth planar surface) and two geometrydependent parameters. In particular, we find that cavities with reentrant walls (i.e., concave interior walls) result in important new conditions. The results show that both thermodynamic equilibrium and metastable states can arise, depending on the geometry (shape) and size of the cavities. We have fabricated some surfaces with micron-scale reentrant features (some of these structures mimic the surface texture of animals, birds, and plant leaves, e.g., lotus leaf, that render them hydrophobic or superhydrophobic), and have tested them with liquids of varying surface tensions, such as water, canola oil, and ethanol. We found that depending on the geometry of the surface features, the apparent contact angles span from the intrinsic angle to values that are very different and are in excellent quantitative agreement with the predictions of our model.

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